Method for heat treatment of a thick steel plate

ABSTRACT

A METHOD OF HEAT TREATMENT OF A THICK STEEL PLATE CHARACTERIZED IN THAT A STEEL PLATE AFTER HOT ROLLING IS COOLED, DIRECTLY FROM ITS FINISHING TEMPERATURE OR AFTER COOLING TO AN OPTIONAL TEMPERATURE BELOW THE AR2 TRANSFORMATION POINT IS REHEATED TO AUSTENITIZE, AND THEN COOLED SLOWLY AT A COOLING RATE OF LESS THAN 20*C. PER MINUTE IN THE RANGE BETWEEN THE AC2 AND AR2 TRANSFORMATION POINTS, AND IS THEN HARDENED BY A USUAL METHOD.

Dec. 11, 1913 SEINOSUKE YANO EI'AL 3,7785'311 METHOD FOR HEAT TREATMENT OF A THICK STEEL PLATE Filed Dec. 29, 1971 5 Sheets-Sheet 1 FIG6 DOC. 11,1973 I SE |NQSUKE YANQ ETAL 3,778,317

METHOD FOR HEAT TREATMENT OF A THICK STEEL PLATE Filed Dec. 29, 1971 5 Sheets-Sheet 2 I O (a) Absorbed Energy kg-m Test TemperutureC Dec. 11, 1973 SEINOSUKE YANO ETAL 3,778,317

METHOD FOR HEAT TREATMENT OF A THICK STEEL PLATE Filed Dec. 29, 1971 5 Sheets-Sheet 3 (d) I: 1 E A (c) 7 A 2O Absorbed A Energy kg-m Test Temperature C Dec. 11, 1973 SEINOSUKE YANO ET AL 3,778,317

METHOD FOR HEAT TREATMENT 01" A THICK STEEL PLATE Filed Dec. 29, 1971 5 Sheets-Sheet 4 2 5 8 9.8m 2% 568 zu nh q g om 0.08 4 mlbmm 25% m wwwm Hm Om uooow o oom METHOD FOR HEAT TREATMENT OF A THICK STEEL PLATE Filed Dec. 29, 1971 1973 SEINOSUKE YANO EI'AL 5 Shasta-Sheet 5 own cum

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United States Patent Ofiice 3,778,317 METHOD FOR HEAT TREATMENT OF A THICK STEEL PLATE Semosuke Yano, Kitakyushu, Hiroshi Sakurai, Kawasaki, Nobuo Wakita, Inagi, and Hiroshi Mimura, Tokyo, Japan, assignors to Nippon Steel Corporation, Tokyo,

Japan Filed Dec. 29, 1971, Ser. No. 213,413 Claims priority, application Japan, Dec. 31, 1970,

123,752 Int. Cl. C21d 7/14 US. Cl. 148-12 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method for the heat treatment of steel plates for the purpose of giving a more superior hardenability at an equal hardening rate. By applying the method of the inventive heat treatment immediately after hot rolling or after a usual austenitizing treatment, it becomes possible to sufliciently harden such a steel plate which cannot be sufliciently hardened using conventional hardening equipment. Particularly the present invention is useful for hardening thick steel plates.

In recent years, the demand for thicker steel plates has been increasing in the field of high tensile strength steels or low-temperature high-toughness steel. However, as these steels are used generally after hardening and tempering treatments, the cooling rate, particularly at the center of the plate decreases with an increase in the plate thickness, resulting in an incompletely hardened structure; thus it becomes diflicult to attain the desired properties such as, a high yield point, a high tensile strength, or an excellent low-temperature toughness.

In order to solve such problems, the addition of various elements capable of increaing the hardenability-in the range where no injury to the desired mechanical properties takes place--has been practiced. On the other hand, improvements in hardening equipment to increase the capacity of hardening have also been made, and equipment for pressure-quench and roller-quench treatments has widely been adopted. However, the results of these methods are not fully satisfactory and such is the present circumstance that, in various grades of steel with a plate thickness of more than 50- mm, a structure uniformly hardened through the outer and inner layers of the plate cannot be obtained. On the other hand, due to various requirements in the equipment or due to deformation during hardening, it is often difficult to carry out the hardening and it is difiicult to obtain a desired hardened structure by air cooling.

The method of heat treatment of this invention is a new method developed from an idea quite apart from the above two methods. By applying the inventive method independently or in combination with one or two of said methods, the objects of hardening can be achieved satisfactorily.

31,778,3 l Patented Dec. 11, 1973 The present invention will be described in detail referring to the attached drawings.

FIGS. 1-3 are optical-microscopic photographs of the steel plates obtained by cooling a steel material with the chemical composition as shown in Table 1 at various cooling rates from the austenitizing temperature. FIGS. 4-6 are optical-microscopic photographs of the steel plates obtained by cooling a steel material with the chemical composition as shown in Table 2 at various cooling rates after hot rolling. FIG. 7 shows impact transition curves for the sample obtained by reheating a steel plate with the chemical composition as shown in Table 2 to austenitize it, hardening and tempering after the usual hot rolling and for the sample obtained by tempering the steel plate corresponding to FIG. 4. FIG. 8 shows impact transition curves for the samples obtained by tempering the steel plates corresponding respectively to FIG. 5' and FIG. 6. FIG. 9 is a graph showing the relationship between the mean cooling rate of the steel plate in the range 550 440" C. and the hardness for samples at various cooling rates after austenitization to 550 C. FIG. 10 is a graph showing the relationship between the duration of holding the steel plate at 600 C. after austenitization and the hardness.

According to the present invention, a steel plate which is obtained by breaking down a steel ingot produced by an ordinary steel making process, hot rolling the ingot and cooling the hot rolled plate to room temperature or a temperature below the Ar transformation point is hardened by reheating above the Ac point. The steel plate is then cooled as slow as possible with a mean cooling rate of less than 20 C. per minute in the metastable austenite region, i.e., between the transformation points A0 and Ar Particularly, the steel plate is held for a suitable periodmore than 3 minutes in the range where no precipitation of ferrite takes place, i.e., at a suitable temperature, defined according to the grades of steel, in the range between the Ac and Ar;, points, and then a usual hardening treatment is carried out. (The treatment will be mentioned hereinafter as K-treatment in general. The treatment to cool the steel plate slowly in the metastable austenite region will be called as K -treatment, and the treatment to hold the steel plate for a definite period at a certain temperature in the metastable austenite region will be called as K -treatment.) By applying such K-treatment, a more well hardened steel plate can be obtained, under the same cooling rate, from a steel plate which cannot be hardened sufiiciently by an ordinary hardening treatment. Further the K-treatment can be applied directly and immediately after hot rolling. Namely, directly after hot rolling is finished, the steel plate is cooled between the transition points Ac and An, as slowly as possible with the cooling rate as above mentioned (K treatment), or the steel plate is held for a suitable period at a suitable temperature between the two transition points (K treatment), and then the steel plate is cooled by a usual method (such as air cooling, oil quenching and water cooling). By applying K-treatment, a more well hardened steel plate can be obtained by the same cooling method as compared with the case in which the steel plate is cooled directly after hot rolling is finished.

When ferrite and pearlite precipitate in the course of slow cooling of the steel plate in the metastable austenite region or holding the steel plate at a suitable temperature in this region, the hardenability is decreased in the present method of heat treatment. Therefore, the steel grade suitable for the present K-treatment is desirously to be the one which contains nickel and whose ferrite precipitation region in the CCT curve is shifted toward the long period side.

Examples of the invention will be set forth below.

FIG. 1 is the optical-microscopic structure (500 magnifications) of the sample obtained by cooling the steel plate with the above mentioned composition at a cooling rate of 80 C. per minute from the austenitizing temperature of 800 C. FIG. 2 is the microstructure (500 magnifications) of the sample obtained by cooling the same steel plate at a cooling rate of 2 C. per minute slowly from the austenitizing temperature of 800 C. to 550 C. and at a cooling rate of 80 C. per minute thereafter. FIG. 3 is the microstructure (500 magnifications) of the sample obtained by cooling the same steel plate from the austenitizing temperature of 800 C. to 600 C., holding for 30 minutes at 600 C. and cooling thereafter at a cooling rate of 80 C. per minute. While the cooling rates of these samples below 550 C. were identical, the sample in FIG. 1 showed an upper bainite structure, and the samples in FIG. 2 (corresponding to K -treatment) and FIG. 3 (corresponding to K -treatment) showed a martensite structure as a whole. The Vickers hardness in FIG. 1 was 293, in FIG. 2 was 339 and in FIG. 3 was 347. FIG. 9 shows the trend of hardness at various cooling rates below 550 C. for the steel plate in this example, the cooling rate in the metastable austenite region being constant, 2 C. per minute or 18 C. per minute. At the cooling rate range of 20 C./min.-200 C./min., corresponding to the condition that the hardening can hardly be carried out when the steel plate is cooled directly from the austenitizing temperature, the effect of slow cooling in the metastable austenite region can distinctly be observed; and the slower the cooling rate in the metastable austenite region, the greater the effect is.

FIG. 10 shows the variation of the hardness by various periods of holding at 600 C. in the metastable austenite region for the steel plate in this example, the cooling rate thereafter being always 80 C. per minute. The hardness begins to increase at a holding period of 2-3 minutes, increases as the holding period is prolonged, and is saturated finally. However, longer holding causes the precipitation of ferrite, naturally decreasing the hardness, and therefore, the upper limit of the holding period should be restrictedalthough different depending on the steel grades-in the range where no ferrite precipitation takes place.

EXAMPLE 2 Table 2.Chemical composition of the steel plate (wt. percent) Fe and impurities Remainder FIG. 4 is the optical-microscopic structure (500 magnifications) of the sample obtained by cooling the steel plate with the above mentioned composition, directly after hot rolling, at a cooling rate of C. per minute from the finishing temperature of 950 C. to 550 C., and under a cooling rate of C. thereafter. FIG. 5 is the microstructure (500 magnifications) of the sample obtained by cooling the same steel plate under a cooling rate of 60 C. per minute directly after finishing the hot rolling (950 C.) and 550 C. and 140 C. per minute thereafter, FIG. 6 is the microstructure (500 magnifications) of the sample obtained by holding the same steel plate for 2 hours at 550 C. in a furnace directly after finishing the hot rolling and cooling under a cooling rate of 140 C. thereafter. While the cooling rates below 550 C. were identical, the K -treated sample (FIG. 5) and the K -treated sample (FIG. 6) showed more complete martensite structure as compared with the sample treated with an ordinary cooling (FIG. 4). The Vickers hardness in FIG. 4 was 315, in FIG. 5 was 318 and in FIG. 6 was 336. The reason Why the improvement of the hardness in FIG. 5 is small is that the cooling rate in the metastable austenite region is 60 C. per minute, much faster than 20 C. per minute.

In FIG. 7, the impact transition curve a of a sample prepared by hot rolling the steel plate as shown in Table 2, reheating to austenitize, and then hardening and temperingis compared with the impact transition curve b of a sampleobtained by tempering, the sample corresponding to FIG. 4. When the steel plate was hardened directly after the hot rolling was finished, the toughness after tempering was not good and the transition temperature was high in general. This is because a sufiiciently hardened structure cannot be obtained in this case owing to requirements in respect of hardening equipments as compared with the case of reheating-hardening and because austenite grains have grown excessively. FIG. 8 shows the impact transition curves 0 and d of the tempered samples corresponding respectively to FIG. 5 and FIG. 6. As the tempering could be done sufficiently, the toughness after tempering was satisfactory, corresponding to the toughness in curve a in FIG. 7.

EXAMPLE 3 Ni 8.3 Mn 0.45

C 0.065 Si 0.28

Al 0.041 N 0.0069

Fe and impurities remainder The Vickers hardness of a usually tempered materialfor its manufacture the steel plate with above mentioned composition is cooled at a cooling rate of 80 C. per minute from the austenitizing temperature of 800 C.- and that of the inventive K-treated sample-for its manufacture the same steel plate is held for 30 minutes at 650 C. in the course of cooling from 800 C. and cooled under a cooling rate of 80 C. thereafterare as follows. The elfect of K-treatment is remarkable.

Usually tempered material: H =314 K-treated material: H =347 What is claimed is:

1. A method for the heat treament of a thick steel plate comprising cooling a steel plate after hot rolling from its finishing temperature to a temperature below the Ar transformation point, then reheating to austenitize, and then cooling the plate at a rate such that it is held in the range between the Ac and Ar transformation points for at least three minutes, and then hardening the plate by the usual method.

2. The method of claim 1 wherein the plate is cooled in the range between the AC and Ar points at an average References Cited 021688 o t h il t Lite} t i th k teel UNITED STATES PATENTS met) e ea ream 0 a S 3,432,368 3/1969 Nakamura 14s -12 plate comprising C g Steel Plate after hot rolling 3,328,211 6/1967 Nakamura 2 from its finlshing temperature at such a rate that is held 5 3,388,988 6/1968 Nagashima et al 148-42 in the range between the Ac and Ar transformation 3 9 02 11 971 Aoki et aL 1 3 12 points for at least three minutes, and then hardening the 3,645,801 2/1972 Melloy t 1. 148 12 plate by a usual method.

4. The method of claim 3 wherein the plate is cooled 10 WAYLAND STALLARD Pnmary Exammer in the range between the Ac and Ar points at an average US 1 rate of less than 20 C. per minute. 148134 

